Automatic frequency control circuit for television receivers



29, 1953 A. COTSWORTH m AUTOMATIC FREQUENCY CONTROL CIRCUIT FOR TELEVISION RECEIVERS Filed June 28, 1951 $500 w: 6260 w: 3536 3533 9205 32 m4 95cm 23:30 m Tl 3536 @3963: xocmscm W m ITT mto0 I W W W 2512 5 m OI m m 85382 MW HI T r e um A E m mi M 3:0 $25225 mm 82 0 vw 2622 nu r. m d 0 Q $52 olo Q o 2.... i r J g Patented Dec. 29, 1953 Albert Cotsworth Oak Park, 111., assignor to Zenith.- Radio corporationya corporation of Illinois Aiiiiiieitidii' J'Ii 2s;19s1, sera na-iitoo 5 oiaisl." (c1. 178-5.8)

'I'h is'inventionreIates to television receivers and moreparticularlyto an impro'ved automatic frequency centrormror circuit for useinZtele-i vi'sio'ifreceivers' of the "intercarrier soun'd type.

Priorto' the advent o'f'present day intercarriersound television receivers, the construction and design of a heterodyne"oscillator with'sufficient frequency stability =forprper operation .of the receiver presented a serious problem; The prob lem arose from thfa-ct" that separation of the video and sound components "of an incoming television signal into their respective intermedF ate-frequency channels" was dependent upon the frequency of the Iheterodyne oscillator and it was necessary "for this frequency to'be rigidly; maintained at a prescribed value." As is wen known, intercarrier-sound. television receivers utilize but a singleintermediate-frequency channel'for both the frequency-modulated sound and amplitude-modulated"video carriers of the received 'televisionsignal, separation between. the' video and sound components-being effected by a. het'erodyne actionbetween these carriers in the second detector which produces an intercarriersignal frequency-modulated with the sound components.. In such receivers; the separation between the video, and; sound components is achieved independently of thewfrequency of the heterodyne oscillation, and the frequencysta-,

loilityv requirementsjof vthe oscillator are nqtas rigorous as in the first-mentioned types of receivers.

The construction j ofj fhe terodyijie oscillators having sumcient freguency stability foruse intercarrier sound television receivers is no part l p m whe such IQQS i E S. rep

in the frequency bands presently allotted'toi tele vision signals. ,However, in the event that the televisionsignal bandisshifted to a higher posi-:

tion inthe frequency spectrum, as is cont'em plated, the problem ,o f frequency s tability i'n" the heterodyne oscillator again' comes to thefore front even in televisionreceiversoff the inter-V This is becau'seiof the e'x.

. .4 involved when receivers are operated in the'com emn f e 'b' i wher n, a smal ipere; 1n thei eratin rre uency or mam; r e fi i' ate al l' quency change in the intermediate freduency r ern ype v tremely high radi aI ld heterodyne-frequencies centage variatio the heterodyne signal of the rece for'automatically controlling the frequencyof the het'e'ro'dyne" oscillator to compensate any. tendency toward drift. Howevensuch a network. may well be proved'essential to a receiver for operation in the contemplated high-frequency band. It has'bee'ncom'mon practicawhen operating'freceivers 'with separate sound and video intermediate frequency channels, to derive for AFC purposes a control signal from the'sounciv discriminator-detector. This was convenient and possible since any shift in the center. frequency 'of the'sound carrier applied to the dis,- criminator represents a related shift in the oper-.

ating 'frequencyof the heterodyne oscillator.

However, in intercarrier-sound receiversthe fre--v quency of the signal applied to the sound detector is in no way related to the frequency of the heterodyne oscillator. since it is merely a beat product 'of the video and sound carriers andhas a center frequencyidetermined solely by the fre-..

quency separation .of these carriers. .For this reason/the sound detectorof an intercarriersound receiver is not an appropriate source of AFCpotential; l

An obvious expedient for the derivation of an AFC control potential would be to providea separate. discriminator circuit coupled to, the. intermediate-frequency amplifier for providing a control signalhaving amplitude variations relatedto frequency shifts of. either the sound or video carriers as translated through this amplifier. In such a case,- the frequency shifts are due to variations in the frequency of the heterodyne oscillator and, therefore, the control signal may be used for AFC purposes. However, such an arrangement entails the inclusion of relatively expensive, additional components and circuitry in the receiver and has been found to be impractical from a commercial standpoint. s

It is, accordingly anpbject of the present invention to provide at a preselected value.

A further objectof thelinvention is toprovide an improved AFC circuitafor an intercarriersound television receiverthatrequires a minimum of relatively inexpensive components and,

therefore, may be constructed cheaply and conveniently.

In accordance with the invention the inter;

mediate-frequency amplifier of a television re, ceiver exhibits a sloping frequency-response characteristic at the frequency Of. the intermediate-frequency sound barrier. This character- A anim r v d AFC i c 'i l qr an intercarrier-sound tele.vision receiver to maln tain the frequency of the heterodyne oscillator mining istic produces amplitude changes in the sound carrier whenever its frequency shifts due to variations in the heterodyne oscillator frequency, and such amplitude changes are reflected in the intercarrier signal derived at the second detector. Theiinvention comprises a rectifier coupled to the frequency-selective circuit utilized to derive the intercarrier signal from the second detector for application to the sound channel. The rectifier produces a unidirectional control potential having amplitude variations related to amplitude variations in the intercarrier signal which, in turn, are indicative of frequency variations in the heterodyne oscillator. A variable reactance unit, preferably of the thermally controlled type, is coupled to the frequency-detercircuit of the heterodyne oscillator, and is controlled by the potential developed in the rectifier to present an effective reactance in accordance with the amplitude of the controlling potential. In this manner, the frequency of the heterodyne oscillator is controlled so that any tendency for it to vary from a selected value is compensated. I

The features of this invention which are believed to be new are set forth with particularity in the appended claims. The invention itself, however, together with further objects and advantages thereof may best be understood by reference to the following description when taken in conjunction with the accompanying drawing, in which:

Figure 1 shows a television receiver incorporating the invention,

Figure 2 is a curve representing the frequency response characteristic of the intermediate-frequency amplifier of the receiver of Figure l, and, Figure 3 is a curve indicating the relation between the amplitude of the intercarrier signal and the frequency of the heterodyne oscillator.

The television receiver illustrated in Figure 1 includes a radio-frequency amplifier I!) having input terminals connected to a suitable antenna ll, 12 and output terminals connected to a mixer amplifier l3. Mixer amplifier I3 is connected to an intermediate-frequency amplifier M which, in turn, is connected to a picture detector 55 through a usual coupling transformer 16. Detector I5 is connected in well-known manner to a first video amplifier I1 and this amplifier is connected through a frequency-selective network l8 to an intercarrier limiter-amplifier stage It. Limiter I9 is coupled to an audio detector 20 of the usual frequency-discriminator type, and the output terminals of detector 20 are connected through an audio amplifier 2| to a sound reproducing device 22.

First video amplifier i1 is further coupled to a second video amplifier 23, the output terminals of the latter amplifier being connected to the input electrodes of an image-reproducing device 24. First video amplifier l! is also connected through a synchronizing-signal separator 25 to a field-sweep generator 26 and to a linesweep generator 21, these generators being connected respectively to the fieldand line-deflection elements 28, 29 of reproducing device 24. An automatic-gain-control circuit 38 has input terminals connected to video amplifier l1 and output terminals connected to stages l0, l3 and M of the receiver, the latter connections being made by means of lead 3|.

The receiver as thus far described is well known to the art, and its operation briefly is as follows: An incoming television signal intercepted by antenna ll, I2 is amplified in radio frequency amplifier i0 and heterodyned to the selected intermediate frequency of the receiver by mixer IS. The frequency-response characteristic of the intermediate-frequency amplifier I4 is shown in Figure 2, and this amplifier translates both the picture and sound intermediatefrequency carriers of the received television signal in accordance with the intercarrier-sound principles. The video information is amplitude modulated on the video carrier and the sound information is frequency-modulated on the sound carrier.

The amplified intermediate-frequency signal is applied to detector l5 wherein the video components are detected, and the video and sound carriers are heterodyned together to produce an intercarrier signal frequency-modulated with the sound components which, by present-day standards, has a center frequency of 4.5 megacycles. The output signals of detector 15 are applied to video amplifier I! wherein they are amplified, the frequency-modulated intercarrier signal being derived by means of frequency-selective network it, and the detected video components being applied to video amplifier 23. The frequency-modulatedv intercarrier signal is amplitude-limited in limiter i9; and the sound components are recovered in detector 20, amplified in amplifier 2i and reproduced by reproducing device 22. The amplified video components from amplifier 23 are impressed on the input elec trodes of reproducing device 24 and control the intensity of the cathode-ray beam therein in well-known fashion. The synchronizing components of the video signal are separated out by separator 25 and used to control the operation of generators 26, 2'! and, therefore, the fieldand line-deflection of the cathode-ray beam of device 24. The automatic-gain-control circuit 30 functions in well-known manner to produce a control signal, related to the intensity of the detected video signal, to control the gain of the high-frequency stages l0, l3 and M to maintain the input signal to detector [5 within a narrow range for a wide range of received signal intensities. In this manner, the image intelligence of the received television signal is reproduced by device 24, and the sound intelligence thereof is reproduced by device 22.

The heterodyne oscillator of the receiver comprises an electron-discharge device 32 having an anode 33 coupled to its control electrode 34 through a capacitor 35 and a frequency-determining network comprising an inductance coil 36 and a shunt capacitor 31. Control electrode 351 is connected to the cathode 38 of device 32 through a network comprising a resistor 39 and a shunt capacitor 48, cathode 38 being connected to ground through a resistor M. Anode 33 is connected to the positive terminal B+ of a source of unidirectional potential through an isolating resistor 42, and is coupled to mixer amplifier [3 through a capacitor 43. The circuit of the heterodyne oscillator is well known, per se and may have any known form. This oscillator supplies a heterodyne signal to mixer is, and the purpose of the present invention is to provide a suitable automatic-frequency control for maintaining the frequency of this heterodyne signal at a prescribed value.

In accordance with the invention, a reactance unit 44 is connected in circuit with the frequency-determining network 36, 31 of the heteressence;

odyne oscillat on thermally.,controlledtype whichexhibits .a .vari:

able; reactance as ;,the. current ..fio,vv through, a thermal, element, included; therein. is varied...

Thermal controlled reaotance. units l are. Well:

known totheart and a detaileddescription of.

the -construction thereof is. deemed to. be. un-.-

necessary. The. unit maybe. ofthe capacitive e. uni srpreferablyoaths type, in which caseitis connected-in shuntwith although a .thermallycontrolled. reactanoe. unit is preferred, any other lrnovvntypeof controllab-1e: .reactance .unit may beoused. The thermal. elementofunit Misconnected. in series with the. positive. terminal. 3+ of.:a source of. .unidirec tional potential andthe anode 4510f a control. tubeAfi, a limitingresistordl also. being. included inthe. anode circuit. The cathode- 48 of control. tube 464s connected to ground, and the.con-' trol electrode d9 of thistubeis connected to. the-anodeof a diode 50 through a filternetwork comprising a series .resistor. diconnectedto. ground through a capacitor 52 picturecarr-ier causes a decrease in its ampli-- tude due to the sloping characteristic of the intermediate frequency' response intermediate-frequency carrier.- The decreased amplitude of the picture carrier causes AGO circuit Soto increase-the gain of the receiver,

tending further to increasethe amplitude of the sound intermediate-frequency carrier. As previously stated, changes-in amplitude ofthe sound carrier are reflected in the amplitude of the intercarrier signal derived by frequency-selective'circuit' l8, this signal being rectified by diode 543 to produce across resistor 53 a uni directional control potential having amplitude variations related in magnitude and sense to amplitude variations of the intercarrier signal from a reference value prevailing when the frequency of the heterodyne oscillatorhas-its correot value. Network*5l,"52 is a filter provided so thatthe AFC circuit does not respond to the rapid, amplitude changes of the sound intermediate-frequency carrier due to its frequency modulation. Accordingly, the variations in the control potential applied to control electrode 49 of 3 tube lfi are related solelyto variations in the amplitude -in-- the sound carrier causedby frequency variations of the heterodyne oscillator. Should the amplitude of the control potential increase, due to an increase in the frequency of the heterodyne oscillator, the space current flowing through device 46 and, therefore, the current through the thermal element of reactance unit 44 decreases. This modifies the effective reactance of device 44 and the resonant frequency of the frequency-determining net- The cathode. of. diodeiie is connected to ground and theanodecurve, but causesan increase in amplitude of the sound 6+ 3.1 o snr' duce. a. qompensatins chansein. he fr u ncr o h heterodyne oscfllatorand restore it to, its...desired.value.: n;.thl 3 other hand, should the. frequency/of; the heterodyne-- sc ato e rease; fr m; ts r per. .va1ue,,,--th

ontro ote a de lop. to pr v de c psnsa i... 2 .9 .5 't '.Q- quen y t eheie o ne' lat r Since-theli u e. f. he nt rmedia efrequ n y: n turecarrier of, all received television signalseismaintained substantially, constant by-the; ACiC circuit, and since the -Peder 1 Communioations Commiss n a res r bed -fixeds.-s andardratio between, the picture and sound; carriers,

.in, a television signal; the. sound carrier tran lated. by, the intermediate{requencyr amplifier has; a fixed; amplitude for allgreceived television, I signals, and. any;v.ariationnin; this amplitude;- (apart from; that due to... e modulationacom-e ponents) indicates. a shift in frequency; of: the heterodyne oscillator. .For this .11eason,;an; ac,cue-- rate controlmay be.maintained omthis oscillator; in accordance. ;.with the, I present; invention for all television :signals; toqwhich ,1 the; receiver; may;.'.=

.be tuned,

It is desirable 'thatgythe frequency. control. Ofg the; heterodyne. oscillator. be; effected by means; of.- I a thermally controlledj: reactancezunih since. such a unit prov-ides. a relatively wide. pull-in; e. fOl the. A-FfC'; Circuit;twithout rendering the oscillator. inherently; unstable, as. is 1 sometimes. the .casewith manyyotherstypescof reactiance units. It. is a simple. mattento: design'rth'er: thermally controlled, reactance unit .1 so thatr has; a relatively high-temperature coefiicientg-t provide. a. sufficientlyrwide pulla-in, range; for all practicalpurposes.

The. invention provides, therefore, an-.,improved;- automatic-frequency. control: .circuitwfor. use; in an intercarrieresound television receivervtorz-piio... vide a precise. control for. the. heterodyne;ioscila. latorgto maintain its frequency at awselectedivaluer with a high degree-of-stability. Thai-AFC circuit:- of the invention is highly efficient :in operation; .1 yet is simple to constructs.andxrequiresxazminie mum of inexpensive, components; 2

Whilea particular; embodiment. of z theainvena r tion has been shown andxa described;.modifications; may be. made and: it is intendeclzin- .theiappended; claims. to cover. alllisuchmodificationsnasiimay fall Withinthe true spiritxandscopeofothe invens tion.

I claim:

1. An intercarrier sound television areceiveri for utilizing. a picture v carrier; amplitude; .modu lated with video. informatiom. and: for .-uconcur.+: rently. utilizing. a sound *carriera having 1: a fixed-l frequency separation- .from:-1 saida-pictur'e carrier and frequency modulated with. sound inform -i: tion, said receiver comprising: a wfirst. detector: including a tunable heterodynmgooscillator for.- converting: said. carriers .xto ;=..-intermediate+fre quency. picture" ands.:sound:'.signals;..: an inter-meg: diate-frequency: channel; coupledl-ato said; :firstxi detector and presenting. air-sloping: frequency;

' response characteristic to said sound signal; a

second detector coupled to said channel for deriving an intercarrier signal of fixed frequency from said picture and sound signals; a frequency-selective rectifier circuit coupled to said second detector for selecting and rectifying said intercarrier signal; a filter circuit coupled to said rectifier to derive therefrom a control potential: substantially independent of said sound infor-- mation and having amplitude .variationsrepra senting frequency variations of said heterodyning oscillator from a reference frequency; and means responsive to said control potential for compensating said frequency variations.

2. An intercarrier sound television receiver for utilizing a picture carrier amplitude modulated with video information and for concurrently utilizing a sound carrier having a fixed frequency separation from said picture carrier and frequency modulated with sound information, said receiver comprising: a first detector including a tunable heterodyning oscillator for converting said carriers to intermediate-frequency picture and sound signals; an intermediate-frequency channel coupled to said first detector and presenting a sloping frequency-response characteristic to said sound signal; a second detector coupled to said channel for deriving an intercarrier signal of fixed frequency from said picture and sound signals; a frequency-selective rectifier circuit coupled to said second detector for selecting and rectifying said intercarrier signal; a filter circuit coupled to said rectifier to derive therefrom a control potential substantially independent of said sound information and having amplitude variations representing frequency variations of said heterodyning oscillator from a reference frequency; and means. including a reactance unit coupled to said heterodyning oscillator, responsive to said control potential for compensating said frequency variations.

3. An intercarrier sound television receiver for utilizing a picture carrier amplitude modulated with video information and for concurrently utilizing a sound carrier having a fixed frequency separation from said picture carrier and frequency modulated with sound information, said receiver comprising: a first detector including a tunable heterodyning oscillator for converting said carriers to intermediate-frequency picture and sound signals; an intermediate-frequency channel coupled to said first detector and presenting a sloping frequency-response characteristic to said sound signal; a second detector coupled to said channel for deriving an intercarrier signal of fixed frequency from said picture and sound signals; a frequency-selective rectifier circuit coupled to said second detector for selecting and rectifying said intercarrier signal; a filter circuit coupled to said rectifier to derive therefrom a control potential substantially independent of said sound information and hav ing amplitude variations representing frequency variations of said heterodyning oscillator from a reference frequency; a thermally controlled reactance unit coupled to said heterodyning oscillater for controlling the operating frequency thereof; and means responsive to said control potential for controlling said reactance unit to compensate and frequency variations of said heterodyning oscillator.

4. An intercarrier sound television receiver for utilizing a picture carrier amplitude modulated with video information and for concurrently utilizing a sound carrier having a fixed frequency separation from said picture carrier and frequency modulated with sound information, said receiver comprising: a first detector including a tunable heterodyning oscillator for converting said carriers to intermediate-frequency picture and sound signals; an intermediate-frequency channel coupled to said first detector and presenting a sloping frequency-response characteristic to said sound signal; a second detector coupled to said channel for deriving an intercarrier signal of fixed frequency from said picture and sound signals; a sound channel coupled to said second detector for selecting said intercarrier signal; a rectifier circuit coupled to said sound channel for rectifying said intercarrier signal; a filter circuit coupled to said rectifier to derive therefrom a control potential substantially independent of said sound information and having amplitude variations representing frequency variations of said heterodyning oscillator from a reference frequency; and means responsive to said control potential for compensating said frequency variations.

5. An intercarrier sound television receiver for utilizing a picture carrier amplitude modulated with video information and for concurrently utilizing a sound carrier having a fixed frequency separation from said picture carrier and frequency modulated with sound information, said receiver comprising: a first detector including a tunable heterodyning oscillator for converting said carriers to intermediate-frequency picture and sound signals; an intermediate-frequency channel coupled to said first detector and presenting a sloping frequency-response characteristic to said sound signal; a second detector coupled to said channel for deriving an intercarrier signal of fixed frequency from said picture and sound signals; an automatic-gain-control circuit coupled to said second detector for controlling the gain of said intermediate-frequency channel in accordance with the intensity of said picture signal; a sound channel coupled to said second detector for selecting said intercarrier signal; a rectifier circuit coupled to said sound channel for rectifying said intercarrier signal; a filter circuit coupled to said rectifier to derive therefrom a control potential substantially independent of said sound information and having amplitude variations representing frequency variations of said heterodyning oscillator from a reference frequency; a thermally controlled reactance unit coupled to said heterodyning oscillator for controlling the operating frequency thereof; and means responsive to said control potential for controlling said reactance unit to compensate said frequency variations of said heterodyning oscillator.

ALBERT COTSWORTH III.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,552,140 Boothroyd May 8, 1951 2,553,368 Breen May 15, 1951 

